A coronary stent is an implantable medical device that is used in combination with balloon angioplasty. Balloon angioplasty is a procedure used to treat coronary atherosclerosis. Balloon angioplasty compresses built-up plaque against the walls of the blocked artery by the inflation of a balloon at the tip of a catheter inserted into the artery during the angioplasty procedure. Unfortunately, the body's response to this procedure often includes thrombosis or blood clotting and the formation of scar tissue or other trauma-induced tissue reactions at the treatment site. Statistics show that restenosis or re-narrowing of the artery by scar tissue after balloon angioplasty occurs in up to 35 percent of the treated patients within only six months after these procedures, leading to severe complications in many patients.
To reduce restenosis, cardiologists are now often placing small tubular devices of various forms, such as wire mesh; expandable metal; and non-degradable and biodegradable polymers called a coronary stent at the site of blockage during balloon angioplasty. The goal is to have the stent act as a scaffold to keep the coronary artery open after the removal of the balloon.
However, there are also serious complications associated with the use of coronary stents. Coronary restenotic complications associated with stents occur in 16 to 22 percent of all cases within six months after insertion of the stent and are believed to be caused by many factors acting alone or in combination. These complications could be reduced by several types of drugs introduced locally at the site of stent implantation. Because of the substantial financial costs associated with treating the complications of restenosis, such as catheterization, restenting, intensive care, etc., a reduction in restenosis rates would save money and reduce patient suffering.
Numerous studies suggest that the current popular designs of coronary stents are functionally equivalent. Although the use of coronary stents is growing, the benefits of their use remain controversial in certain clinical situations or indications due to their potential complications. It is widely held that during the process of expanding the stent, damage occurs to the endothelial lining of the blood vessel triggering a healing response that re-occludes the artery. To help combat that phenomenon, drug-coated stents are being introduced to the market to help control the abnormal cell growth associated with this healing response. These drugs are typically mixed with a liquid polymer and applied to the stem surface. The polymer coating can include several layers such as the above drug containing layer as well as a drug free encapsulating layer, which can help to reduce the initial drug release amount caused by initial exposure to liquids when the device is first implanted. A further base coating of polymer located beneath the drug bearing layer is also known. One example of this arrangement used on stainless steel stems includes a base layer of Paralene C, and a drug/polymer mixture including polyethylene-co-vinyl acetate (PEVA) and poly n-butyl methacrylate (PBMA) in a two to one ratio, along with a non-drug impregnated top layer of the same mixture of PEVA and PBMA. The drug used is Sirolimus, a relatively new immunosuppressant drug also known as Rapamycin. Several other drug/polymer combinations exist from several manufactures.
In view of this new approach to in situ drug delivery, it is desirable to have greater control over the drug release rate from the implantable device as well as control over other surface characteristics of the drug delivery medium.
It is therefore an object of this invention to provide a means of controlling surface characteristics of a drug eluting material using gas cluster ion beam technology.
It is a further object of this invention to improve the functional characteristics of known in situ drug release mechanisms using as cluster ion beam technology.